1. Field of the Invention
The present invention relates to a forward converter having an active clamp circuit, selected from switching power sources, that does not require any insulating element such as pulse transformer or the like.
2. Description of Related Art
FIG. 3 shows the circuit configuration of a forward converter with a conventional active clamp circuit.
As shown, a transformer 107 has a primary coil 108 connected at one end thereof to a positive pole of an input DC source 106, and at the other end to a negative pole of the input DC source 106 via a main switching element 101.
Also, a series circuit consisting of an auxiliary switching element 102 and capacitor 103 is connected in parallel to either end of the primary coil 108 of the transformer 107. Thus an active clamp circuit is formed.
The transformer 107 has a secondary coil 109 to which there is connected in parallel a rectifier circuit consisting of a rectifying diode 111 and a commutating diode 112, and a smoothing circuit consisting of a reactor 113 and a capacitor 114, thus forming a choke input type rectifier circuit.
There is provided a control circuit 104 to turn on and off the main and auxiliary switching elements 101 and 102 alternately. The control circuit 104 detects a voltage at a point A of the secondary output circuit to control the auxiliary switching element 102 via a pulse transformer 105 based on the detected voltage. The main switching element 101 is controlled directly by the control circuit 104, not via the pulse transformer 105.
In a forward converter using the above-mentioned active clamp circuit, the main switching element 101 has a drain and source between which there develops a voltage having a waveform V.sub.ds, as shown in FIG. 4(a). As shown, the voltage has a square waveform (E.sub.i +V.sub.ccl) where E.sub.i is an input DC source voltage and V.sub.ccl is a voltage applied to the capacitor 103.
As seen from FIG.3, to drive the auxiliary switching element 102, it is necessary to provide an isolation between a gate signal supplied from the control circuit 104 to the main switching element 101 and a gate signal to the auxiliary switching element 102. To this end, the pulse transformer 105 is provided in the gate circuit of the auxiliary switching element 102.